1.0.15.2: Add binding to fcntl's struct flock in SB-POSIX.
File by file breakdown:
* contrib/sb-posix/constants.lisp
Add an internal ALIEN-FLOCK structure.
* contrib/sb-posix/defpackage.lisp
Export the FLOCK class the accessors.
* contrib/sb-posix/interface.lisp
Define a FLOCK "protocol class", add a path through FCNTL that accepts
FLOCK instances.
* sb-posix/posix-tests.lisp
Test two uses of exclusive (F_WRLCK) locks.
* contrib/sb-posix/sb-posix.texinfo
Add documentation for the FLOCK class to the manual.